Fans of The Wire will remember a deathless scene where the policemen extract a confession from a murder suspect with a homemade lie detector. First, they load a photocopier with three sheets of paper reading TRUE, TRUE and LIE; they bring in the suspect and ask him three questions. Is your name X? Do you live in Baltimore? Did you kill your friend Rashid? TRUE, TRUE, LIE. The suspect is horrified, and starts blabbing.

Actually, real lie detectors are hardly any more reliable than this, and most psychologists who have looked into it regard the polygraph's measuring of blood pressure and perspiration as pseudoscience or, at best, an interrogation tool with unacceptable levels of error in either direction. Only in a very few places are the conclusions of a lie detector accepted as evidence in court; over here, I think only the Jeremy Kyle show takes it seriously.

So it's amazing that the Government is pressing for the introduction of compulsory polygraph tests on sex offenders. Two pilot programmes in the Midlands between 2009 and 2011 are being wheeled out across the country.

It is claimed that subjecting sex offenders to these tests will encourage them to be more honest, to behave better and get them back on the right track. That may be so, but we ought to be clear that, like homeopathy, lie detector tests will work best if the victim believes that they work. If the polygrapher says, "This can tell whether you are lying or not," and the offender believes him, he is more likely to tell the truth.

Using Heath Robinson devices to persuade people to be more truthful is harmless, I suppose, so long as nobody starts thinking that the machine must be right if it says a person is lying.

Still more worrying would be if the results of these devices started to be admitted in evidence. That already happens in some American states. If it spread, it would be a recipe for endless miscarriages of justice.

Joy of an imperfect Proms performance

At the Proms this week, the soprano Anne Schwanewilms looked a little out of sorts as soon as she came on to sing Strauss's Four Last Songs with the BBC Philharmonic. She took her jacket off after the first song, evidently feeling faint. Her voice was not strong. And then in the third song, things went terribly wrong. She started the famous ecstatic conclusion in the wrong octave, realised her mistake, switched back to the right place with a yodel, then broke down altogether and stood there for a moment, smiling, trying to work out where she was.

She's a wonderful singer – we heard her in recital in Geneva recently, and she was unforgettably good – and I'm sure this was a one-off. It's interesting, however, how harsh some of the reaction was – "awful, gedderoff", "Seriously ballsed up", "massacre", "effortless … apart from forgetting words and music".

But in the past, people have expected and overlooked slips, errors, blunders of all sorts in performances of music.

What is being expected here is perfection, and when a performer falls short of perfection, the audience is quick to voice its dissatisfaction.

In pop music, the situation is a little more ambivalent. Audiences at Amy Winehouse's or Whitney Houston's last concerts didn't expect to hear CD-standard performances. They wanted to see disarray, the human being showing herself through former glories. You want perfection? Go home and put the CD on.

In life, there's a gap between perfection and the acceptable which we fill, most of the time. It's one of the strange facts of life that real perfection, effortlessly achieved, can be rather disappointing for the onlooker. We don't have the right to expect perfection, and we wouldn't like it if we got it.

Schwanewilms's performance had moments of great beauty, some ragged edges and one big hole. I expect it was pretty well what most good professional performances were like a century ago, though an unusual experience at the Proms these days.

If you want to hear perfection in this piece, there's a great recording by Klaus Tennstedt and Lucia Popp; if you are open to the idea of human frailty shaping a performance, then it was possible to be moved by Anne Schwanewilms the other night. More wrong notes! More forgotten lines! And then we might properly appreciate perfection when it comes.

A toast to royal relics

Someone has paid £230 at auction for a 30-year-old piece of toast. Not just any old piece of toast, however – a piece of toast served to, but not eaten by, Prince Charles on the morning of his wedding to Diana Spencer in 1981. The provenance was impeccable.

What an extraordinary, moving bit of carbohydrate. The scene constructs itself: the young Prince, about to reach for his usual second piece of toast, but suddenly overcome with doubt; the toast, returned, untouched, puzzled over by the staff; and finally one says, "That toast, colleagues, is history. In that toast, we shall see this marriage unfold. I am keeping that, to sell it at a profit in decades to come." The age of the reliquary is not over. God knows what it would have fetched if the royal toast had been proffered to the sainted Diana herself, or if evidence of marital doubt were present in a single, uncommitted bite mark.

Chris Bryant is away